1. Field of the Invention
The present invention relates to a bi-directional optical module able to transmit and receive optical signals for a single fiber, in particular, the invention relates to an optical module that installs a light-transmitting device and a light-receiving device within a single package.
2. Related Background Art
The metro-optical communication system has bee developed and become popular. This system, for using already existing optical fiber effectively, applies an arrangement where the optical transmission and the optical reception are carried out for the single fiber by differentiating the signal wavelengths. That is, the optical transmission is carried out in a wavelength of 1.31 μm, while, the optical reception is in a wavelength of 1.49 μm or 1.55 μm. These configurations are directed to the digital signals. Recently, further request to transmit an analogue signal in addition to those digital signals has been occurred. Such a system transmits the analog signal in the wavelength of 1.55 μm, while, the digital receiving signal is assigned to the wavelength of 1.49 μm.
A bi-directional optical assembly (hereafter denoted as BOSA) or an optical module applied in such communication system had an arrangement that assembles separate packages each for the optical transmitting device and the optical receiving device with the wavelength division multiplexing filter (hereafter denoted as WDM filter). Such an arrangement, although the devices are able to be developed and generalized independently, is necessary to assemble respective devices with the WDM filter. Moreover, the individually packaged configuration inevitably results in the large sized housing of the module.
A modified arrangement has been proposed to solve the subjects above, where a light-emitting device, typically a semiconductor laser diode (hereafter denoted as LD), and a light-receiving device, typically a photodiode (PD), are housed within a single package. The U.S. Pat. No. 7,093,988, has disclosed an example of such a bi-directional module with single package. This optical module installs an LD and a PD within the single co-axial package. The LD emits light in a direction parallel to the stem of the package. This light couples with the optical fiber attached with an optical ferrule in the end thereof after it is reflected by substantially a right angle by the WDM filter to a direction perpendicular to the stem of the package and concentrated by the lens.
On the other hand, light emitted from the optical fiber may couple with the PD mounted on the stem after it is concentrated by the lens and passes through the WDM filter. Thus, the WDM filter is an optical device that reflects the light emitted from the LD while passes the light provided from the optical fiber to couple with the PD.
This bi-direction module with the single package inherently has a subject of the optical isolation and the electrical isolation between the transmitting unit and the receiving unit. That is, the LD in the transmitting unit emits the modulated light synchronized with the switching current supplied thereto. To turn off and on a large current inevitably induces the electro-magnetic interferential noise. On the other hand, the PD converts a faint optical signal, the optical power of which is occasionally smaller than a several hundreds micro-watts, into a corresponding electrical signal typically smaller than several mV; thus, the EMI noise caused by the switching current affects the receiving condition of the PD.
Moreover, an optical isolation deteriorates by the diffusive reflection of the light emitted from the LD. Specifically, the light emitted from the LD may couple with the PD after the diffusive reflection within the package. Because the PD generally shows the optical sensitivity in wavelengths of the laser light, this diffusively reflected light degrades the optical isolation of the module. An example shown in the U.S. Pat. No. 7,093,988 provides an additional conductive cap such that it covers the PD and the pre-amplifier which amplifies the faint signal provided from the PD. The cap provides a window, while, the WDM filter is set so as to seal this window. Thus, the cap may electrically isolate the PD and the pre-amplifier from the LD, and concurrently, the cap may optically isolate the PD from the diffusively reflected light from the LD. However, the module shown in U.S. Pat. No. 7,093,988 is necessary to prepare this additional cap independently from the package cap and to widen the stem area because two caps are welded with the stem. The conventional module provides a sleeve also assembled with the stem, which further requests the package to enlarge the size of the stem.
The present invention is to provide an arrangement, in a bi-directional optical module with a type of a single package that realizes both the electrical and the optical isolation between the transmission unit and the reception unit without enlarging the package size.